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WHITTAKER PERIODS, MOTIVIC PERIODS, AND SPECIAL VALUES OF TENSOR PRODUCT $L$-FUNCTIONS

Published online by Cambridge University Press:  31 March 2015

Harald Grobner
Affiliation:
Fakultät für Mathematik, Universität Wien, Oskar–Morgenstern–Platz 1, A-1090 Wien, Austria ([email protected])
Michael Harris
Affiliation:
Univ Paris Diderot, Sorbonne Paris Cité, UMR 7586, Institut de Mathématiques de Jussieu-Paris Rive Gauche, Case 247, 4 place Jussieu F-75005, Paris, France Sorbonne Universités, UPMC Univ Paris 06, UMR 7586, IMJ-PRG, F-75005 Paris, France CNRS, UMR7586, IMJ-PRG, F-75013 Paris, France Department of Mathematics, Columbia University, New York, NY 10027, USA
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Abstract

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Let ${\mathcal{K}}$ be an imaginary quadratic field. Let ${\rm\Pi}$ and ${\rm\Pi}^{\prime }$ be irreducible generic cohomological automorphic representation of $\text{GL}(n)/{\mathcal{K}}$ and $\text{GL}(n-1)/{\mathcal{K}}$, respectively. Each of them can be given two natural rational structures over number fields. One is defined by the rational structure on topological cohomology, and the other is given in terms of the Whittaker model. The ratio between these rational structures is called a Whittaker period. An argument presented by Mahnkopf and Raghuram shows that, at least if ${\rm\Pi}$ is cuspidal and the weights of ${\rm\Pi}$ and ${\rm\Pi}^{\prime }$ are in a standard relative position, the critical values of the Rankin–Selberg product $L(s,{\rm\Pi}\times {\rm\Pi}^{\prime })$ are essentially algebraic multiples of the product of the Whittaker periods of ${\rm\Pi}$ and ${\rm\Pi}^{\prime }$. We show that, under certain regularity and polarization hypotheses, the Whittaker period of a cuspidal ${\rm\Pi}$ can be given a motivic interpretation, and can also be related to a critical value of the adjoint $L$-function of related automorphic representations of unitary groups. The resulting expressions for critical values of the Rankin–Selberg and adjoint $L$-functions are compatible with Deligne’s conjecture.

Type
Research Article
Copyright
© Cambridge University Press 2015 

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